Method of making lead arsenate



' UNITED STATES.

PATENT OFFICE.

EDWIN o. B Its'roW, OF MIDLAND, n cIIIG-AN, ASSIGNOR TO THEDOW CHEMICAL COMPANY, on MIDLAND, MICHIGAN, A CORPORATION or MICHIGAN.

' METH D OF MAKING LEAD AnsE Arn Application filed June 28, 1918. Serial No. 242,336

, To all whom it may concern:

I Be it known that I, EDW BARs'row, a citizen of the United States, and a resident of Midland, county of Midland, and State,-

of Michigan, have invented a new and useful Improvement in Methods of Making Lead Arsenate, of which the following is a specification,the principle of the invention being herein explained and the best mode in which I have contemplated applying that principle, so as'to distinguish it from other inventions;

It is known that arsenic acid will precipitate as an arsenate the lead contained in the sub-acetate or basic lead acetate. The object of the present invention is to provide a process for the utilization of this reaction in the manufacture of lead arsenate on a commercial scale. A further object isto provide acontinuous process,-that is one in i which the several materials utilized may be supplied continuously at proper stages in the process and the desired lead arsenate withdrawn at the end. The advantage of such a process over those currently in use, in

which arsenate is made up in batches, will be obvious.

To the accomplishment of the foregoing and related ends the invention then' consists of the steps hereinafter fully described and particularly pointed out inthe claims,

thefollowing description and the annexed drawing setting forth in detail certain steps embodying the invention, such described steps constituting, however, but one of ,variou's ways in which the principle of the in vention may be used.

In said annexed draw1ng:-- I Figure 1' 1s a diagrammatic representatlon of anapparatus arranged for carryingout' my present improved process; and Fig. 2 is a diagrammatic representation of theseveral steps involved in such process.-

eferring more especially to the apparatus as'outlined in Fig. 1, the first step'of theprocess consists in continuously mixing acetate solutionis returned to the stage in question from a subsequent step in the rocass as will 'be presently described hese two ingredients are preferably supplied to thebottom of,the chamber 1 where suitable stirring means 2 serves to thoroughly, intermix the same, and as the level of the liquid in the chamber rises, such liquid will overv flow into second reaction chamber 3 likewise supplied with suitable stirring means 4.

The solution'thus' overflowing into chamber 3 contains principally basic lead acetate in solution and arsenic acid is here supplied to such' solution from an outside source, the

result being the formation of a precipitate of lead arsenate, leaving a solution ofilead acetate and free acetic acid.

The mixture of lead acetate and free acetic acid solution with precipitated lead arsenate is conducted from chamber 3 to a suitable settling tank 5 from the upper portion of which such solution, practically clear, is withdrawn and pumped back to chamber 1, thus supplying the lead acetate solution there needed; The lead arsenate which set ties as a sludge in the bottom of chamber 5, is pumped to a mixing box 6, where it is mixed with water, and then overflowing into a second settling tank 7, is again separated out as a sludge and finally pumped to a filter 8. The overflowing liquor from tank 7' 4 It Will be understood that any number of mixing. boxes and settling chambers may be utilized in washing the lead arsenate after its separation from the solution in settling tank 5, inste'ad of the single such mixing box and chamber illustrated; also that in stead of having a singl reaction chamber 3,

the reaction between the arsenic acid and basic lead acetate solution may be continued through two or more similar chambers. It

may also be found desirable to concentrate the basic lead acetate solution which over flows'from reaction chamber ,1,- dependlng upon its degree of dilution. Suitable evaporating means may accordin ly be. interposed betweensaid reaction cham er and chamber 8, in which the arsenic acid is added, to such basic lead acetate solution.-

By the succession of steps-just described including the continuous return of the lea acetate and acetic acid solution from the sec thoarsenate, Pb (AsO,,)

0nd step to the first, the addition of lead acetate from an extraneous source is unnecessary except in starting the process and in It will be noted that the foregoing equations are based on the assumption that the simplest basic salt (PbOH.C H O 1s formed as a result of the second reaction. and that the arsenate produced is the or- The commercial product, however, may contain other arsenates, just as other basic acetates may be formed by the first reaction, without affecting thecharacter or success of the process.

Other modes of applying the principle or ,my invention may be employed instead or" the one explained, change being made as regards the method herein disclosed. provided the step or steps stated by any of the following claims or the equivalent of such step or steps be employed.

I therefore particularly point out and claim as my invention In a method of making lead arsenate, the steps which consist in maintaining a mix: ture of lead oxid and lead acetate, the former being present in excess; continuously separating out the basic lead acetate thus formed: and continuously mixing such basic lead acetate solution with arsenic acid, whereby lead arsenate is precipitated.

".3. In a method of making lead arsenate, the steps which consist in maintaining a mix: ture of lead oxid and lead acetate, the forher being present in excess; continuously separating out the basic lead acetate thus formed: continuously mixing suchbasic lead acetate solution with arsenic acid, whereby ead arsenate is precipitated, leaving free lcetic acid: and continuously separating and :eturning such acetic acid,t0 the first step to .eact with lead oxid there and form more .ead acetate.

3. In a method of making lead arsenate, 611B steps which consist in continuously mixing iead oxid and lead acetate solution containing free acetic acid in a suitable reaction chamber: continuously drawing oil from such chamber the solution of basic lead acetate that is thus formed; continuously mix- ;ng in a separate reaction chamber such basic lead acetate solution with arsenic acid, whereby lead arsenate is precipitated, leaw ing lead acetate with free acetic acid; passing the resulting mixture through asettling chamber: continuously withdrawing such lead arsenate from the bottom of such settling chamber: and continuously withdraw- .ng such acetic acid from the top of such settling chamber and returning the same to the irst reaction chamber.

Signed by me. this 20th da of June 1918.

EDWIN O. ARSTOW. 

